A magnetic recording medium is widely used, for example, as a recording tape, a video tape, or a floppy disc. Magnetic recording media generally comprise a non-magnetic support and a magnetic layer arranged on the support, the magnetic layer comprising a binder and a ferromagnetic powder dispersed therein.
Magnetic recording media are evaluated with respect to electromagnetic conversion characteristics, running durability and running performance. For example, an audio tape for recording and reproduction of music desirably has a high sound reproducing ability. On the other hand, a video tape desirably has excellent electromagnetic conversion characteristics such as original picture reproducing ability.
In addition to the above described electromagnetic conversion characteristics, the magnetic recording medium desirably has good running durability. The running durability is generally provided by use of an abrasive and a lubricant.
However, if it is intended to obtain the desired running durability using an abrasive, it is necessary to increase the content of the abrasive in the magnetic layer. However, an increase in the content of the abrasive decreases the content of the ferromagnetic powder in the magnetic layer. When an abrasive having a large particle size is used to obtain excellent running durability, the abrasive tends to diffuse to the surface of the magnetic layer. Thus, in many cases, the improvement in the running durability by using an abrasive deteriorates the electromagnetic conversion characteristics.
Also, when the running durability is enhanced by using a lubricant, it is necessary to increase the content of the lubricant in the magnetic layer. An increase in the content of the lubricant plasticizes the binder, and the durability of the magnetic layer tends to decrease.
As another method for improving the running durability, a hard binder has been used to increase the hardness of the magnetic layer. An increase in the hardness of the magnetic layer is disadvantageous because the magnetic layer becomes markedly fragile, drop out results by contact with a magnetic head, and other characteristics (e.g., durability during still) are deteriorated.
In addition, a magnetic recording medium comprising polyisocyanate, a polyurethane resin containing a hydroxyl group and having a molecular weight of less than 10,000, and a resin having an active hydrogen atom is disclosed in JP-A-58-153224 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). The polyurethane resin (or other resin) having an active hydrogen atom (for example, a hydroxyl group) reacts with polyisocyanate (cross-linking agent), to provide a magnetic layer which is very stiff. Thus, a magnetic layer is obtained which exhibits less decrease in output even when stored for a long time, and which has good durability. However, the above described magnetic layer is unsatisfactory with respect to running durability because the adhesion of the magnetic layer to the non-magnetic support is poor.
As a technique for improving both the electromagnetic conversion characteristics and the running durability, a magnetic recording medium is proposed in JP-A-63-103429 comprising two magnetic layers provided on a non-magnetic support, said layers containing a ferromagnetic powder dispersed in a binder and having a coercive force of not less than 500 Oe, wherein the lower magnetic layer (first magnetic layer) disposed closest to the non-magnetic support has a Young's modulus of 500 to 1,000 kg/mm.sup.2 and the upper magnetic layer (second magnetic layer) has a Young's modulus of not less than 1,300 kg/mm.sup.2. Furthermore, the Young's modulus of the entire magnetic layer comprising said first and second magnetic layers is at least 900 kg/mm.sup.2. According to the disclosure of JP-A-63-103429, the lower magnetic layer is soft such that it has a high buffer action and high adhesive properties, and the upper magnetic layer is increased in hardness such that it is less deformed even when stored under high temperature conditions. Furthermore, the magnetic recording medium is said to have improved running durability.
For obtaining the above described Young's modulus of the first and second layers, a number of techniques can be employed, including a method wherein substantially equal amounts of the ferromagnetic powder are used, and the first magnetic layer (lower layer) employs a binder having a lower Young's modulus than that used in the second magnetic layer (upper layer); a method wherein equal amounts of binder are used, and the amount of the ferromagnetic powder contained in the first magnetic layer is less than that contained in the second magnetic layer; and a method wherein both the amount of the ferromagnetic powder and the amount of the binder are controlled, to thereby obtain the desired Young's modulus of the first magnetic layer and the Young's modulus of the second magnetic layer.
However, JP-A-63-103429 only discloses a technique for obtaining the desired Young's modulus using the same binder in both the first and second magnetic layers, by changing the relative amount of a vinyl chloride-vinyl acetate-vinyl alcohol copolymer component (i.e., a relatively hard binder component) to the amount of a polyester-polyurethane (i.e., a relatively soft binder component). With the above described combination, however, in the lower magnetic layer (first magnetic layer) having a smaller Young's modulus, the amount of the vinyl chloride-based copolymer having relatively good dispersibility is decreased, and the amount of the polyester-polyurethane having poor dispersibility is correspondingly increased, such that the overall dispersibility of the lower layer is markedly reduced. As a result, the upper layer (second magnetic layer) exhibits poor surface properties and low strength. On the other hand, in the present invention, the glass transition temperature Tg of the polyurethane component of the binder is controlled to provide both excellent electromagnetic conversion characteristics and running durability.